Technical Abstract:
Bioplastic was prepared from cornstarch or poly(hydroxybutyrate-co- hydroxyvalerate) (PHBV) or blends thereof. Blended formulations incorporated 30% or 50% starch in the presence or absence of polyethylene oxide (PEO). Degradation of these formulations was monitored at four stations in coastal water southwest of Puerto Rico. Two stations were within a mangrove stand. Two were sited offshore, of which one station was on a shallow shoulder of a reef and one was at a location in deeper water. Microbial enumerations of the four stations revealed considerable flux in populations. However, overall population densities were an order of magnitude less at the deeper water station. Starch-degraders were 10 to 50-fold more prevalent than PHBV-degraders. Accordingly, the degradation of bioplastic correlated with the amount of starch present (neat starch > 50% starch > 30% starch > neat PHBV). Incorporation of PEO into blends had little effect on the rate of degradation. The rate of starch loss from neat samples was about 2%/day, while neat PHBV was lost at a rate of about 0.1%/day. A predictive mathematical model for individual polymer loss was developed and experimentally validated. The model showed that PHBV degradation was delayed 50 days until over 80% of the starch was consumed and predicted that starch and PHBV in the blend had half-lives of 19 and 158 days, respectively. In agreement with the relatively low microbial populations, bioplastic degradation at the deeper water station exhibited an initial lag period, after which degradation rates comparable to the other stations were observed. It can then be reasonably inferred that extended degradation lags would arise in open ocean waters where microbes are sparse.